The 5th is aquired through an event that takes place once every 4 hours on the following time table:

24:00 - 04:00 - 08:00 - 12:00 - 16:00 - 20:00 etc

It takes place in Brill, Falconwing Square and Razor Hill for Horde or the respective secondary starting villages for Alliance (Goldshire, Kharanos etc.)

The "questline" goes as follows:
You take the quest given by any 2 random Commoners in any major city; this leads you to one special NPC (Costumed Orphan Matron) that will give you a quest ( http://thottbot.com/q11439 ).

After that, you have to wait for the 1-in-4-hour event and a new quest pops over the Matron. It will tell you to put out the fires that the flying Level 11 Headless Horseman is throwing on buildings. When all the fires are taken care of, he descends and he must be killed. Once dead, you can click a giant pumpkin that is his head, and you receive [Smashed Pumpkin Loot] ( http://www.wowhead.com/?item=34077#comments ).

-This box can contain the 5th type of broom ( http://www.wowhead.com/?item=33189 ). One that has no Level requirement but runs at about the same speed as normal running. It's great for showing off with alts or banks.-
The box doesn't necessarily drop the broom.

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!!! ALL BROOMS LAST 14 DAYS. Although the green text reads: "Good for only one ride", you can use them as often as you like until it despawns from your backpack after the 14 days. !!!
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After the event is finished, and the quest is turned in, you are sent to Scarlet Monastery - Graveyard for the Level 70 one (if you have the required level of course).

This event can be found in SM-GY in the "gardens" before the entry to the catacombs inside the istance.
There is a big pumpkin that can be clicked.

It holds a quest that gives you a candle that you need to place on the pile of dirt behind the pumpkin. Be sure to clear around the area of the fight before-hand, because extra adds will make it difficult to target the head once it spawned.

The rest of the tactics, loot and technical information can be found here: